1. Field of the Invention
The present invention relates to an acceleration detecting system for rotating bodies of the type which may for example be incorporated advantageously in an anti-skid control system for preventing an irregular gyration of a vehicle which results when the rotation of the wheels is stopped by an excessively large braking force during the braking of the vehicle.
2. Description of the Prior Art
For example, anti-skid control systems for vehicles are known in the art which comprises a generator for generating a DC voltage corresponding to the rotational speed of a wheel, a differentiation circuit for generating an output voltage only when the DC voltage changes suddenly, and a braking force control unit for forcibly releasing the braking force applied to the wheel only when the differentiation circuit generates an output voltage, whereby when the rotation of the wheel decreases rapidly so that there is a tendency for the wheel to lock, the braking force is released to prevent the locking of the wheel.
However, due to the fact that the sudden deceleration of the wheel rotation is detected in terms of a variation of the DC voltage or in terms of a negative acceleration obtained by differentiating the wheel speed, the anti-skid control system of the above type has a disadvantage and any attempt to overcome this disadvantage inevitably tends to make the system considerably high in cost and complicated in construction. In other words, though small in quantity, the output voltage of the generator includes a ripple component. That is, even the output voltage of a DC generator or the output voltage obtained by rectifying and smoothing the output voltage of an AC generator necessarily contains a ripple component and therefore the output voltage varies due to the ripple component even when the rotation of the wheel is smooth, causing an erroneous operation of the system in which the differentiation circuit is caused by the ripple component to erroneously generate an output voltage and the braking force control unit is energized to release the braking force.
While, in an attempt to overcome the foregoing difficulty, the number of poles in a generator may be increased to reduce the ripple voltage, this has the effect of making the generator bulkier and heavier. In this case, if the size and weight of the generator are not increased, the output voltage is decreased so that the superposition of external noise or the like on this DC output voltage may cause the system to operate erroneously. Further, if a capacitor or the like is connected to the generator to smooth the DC output voltage to a greater extent, contrary to the above-mentioned case, when the rotation of the wheel actually decreases suddenly, the resultant DC output voltage fails to follow this sudden deceleration so that at the expiration of a considerable time following the sudden deceleration of the wheel, the differentiation circuit generates an output voltage to release the braking force. This has a very detrimental effect so that in an extreme case the braking force will be released only after the stoppage of the wheel rotation.
There is another disadvantage that in order to detect variations of the DC voltage the DC voltage must always assume a value proportional to the rotational speed of the wheel, otherwise the same time rate of change or the angular acceleration of the wheel rotational speed shows a difference between the high wheel rotational speed and the low wheel rational speed. Also, with the differentiation circuit, if its output is allowed to vary with the magnitude of the DC voltage, it gives rise to a difficult situation in which the rate of change of the DC voltage or the time rate of change (acceleration) of the wheel rotation cannot be detected accurately. While all of these difficulties arise from the fact that the rotational speed of a wheel is converted into a DC voltage which is an analog quantity and this DC analog voltage value is then differentiated, any attempts to overcome these difficulties by using a generator which operates with a high degree of accuracy, that is, a generator which is capable of always generating the same output voltage for variations in the ambient temperature and the like and by reducing the variations in the performance of individual elements of the same type caused during the manufacturing process, inevitably have the disadvantage of making the system expensive. Also with the differentiation circuit, the operating characteristics of its component elements such as transistors vary considerably with temperature changes and their characteristics also vary with variations of the power supply voltage. Consequently, it is necessary to use many component elements such as a temperature compensating circuit, a voltage compensating circuit, etc., which are necessary for controlling the variations of the characteristics below the predetermined values, but which have no direct bearing on the essential operation of the system, that is, there are also the disadvantages of making the system expensive, unnecessarily complicated in construction and difficult to operate.